CCD Triple-split gate electrode transversal filter

ABSTRACT

By dividing the split gate electrode into four sections, in which two non-adjacent sections are solely clock sections and the other two sections are &#34;plus&#34; and &#34;minus&#34; sections, respectively, and in which the sum of the lengths of the first two sections, in serial order, is substantially equal to the sum of the last two sections, in serial order, only a single mask must be changed, in the fabrication of transversal filters that exhibit a small common mode, in order to change the particular filter characteristic of a fabricated filter.

This invention relates to a charge-coupled device (CCD) split gateelectrode transversal filter and, more particularly, to one havingimproved architecture.

Reference is made to the article "Double-Split-Electrode TransversalFilter For Telecommunication Applicants," by Ibrahim, Hupe and Foxall,appearing in the IEEE Journal of Solid-State Circuits, Vol. SC-14, No.1, February, 1979. The advantage over previous CCD split gate electrodefilters of the technique described in this article is that thecommon-mode signal is reduced and minimized. This reduces therequirements on the sense amplifiers. However, the disadvantage of thearchitecture of the filter described in this article is that itsfabrication requires a different set of three masks for each differentfilter design.

The architecture of the CCD split-gate-electrode transversal filter ofthe present invention retains all the advantages of thedouble-split-electrode transversal filter described in the aforesaidarticle. In addition, however, the architecture employed by the filterof the present invention has the further advantage of requiring a changein only a single mask for the fabrication of each filter design havingdifferent filter characteristics.

More specifically, both the CCD split gate electrode transversal filterdescribed in the aforesaid article and the CCD split gate electrodetransversal filter of the present invention are operated by multi-phaseclock voltages, and both filters are of the type comprised of asemiconductor substrate surface on which pair of substantiallyequidistant potential barriers defines a CCD channel of a given width.Gate electrodes of the filter extend the given width between thepotential barriers, with the gate electrodes being arranged inrespective sets corresponding to each of the multi-phase clock voltages.Respective members of each set are distributed along the length of thechannel in interleaved relationship with respective members of the othersets. Certain members of a certain one of these sets are each split intoa plurality of separate sections arranged in serial order across thegiven width of the channel. A "plus" summing bus electrically connectstogether first corresponding sections of the members of this certain oneof the sets, and a "minus" summing bus electrically connects togethersecond corresponding sections of the certain members of this certain oneof the sets. However, in accordance with the improved architecture ofthe CCD split gate electrode transversal filter of the presentinvention, the filter includes at least one triple-split gate electrodecomprised of four serially oriented longitudinal sections. The sum ofthe lengths of the first two sections in serial order is substantiallyequal to the sum of the lengths of the last two sections in serialorder. Further, first means are provided for operating two non-adjacentones the four sections as solely clock sections and second means areproviding for operating the other two of the four sections,respectively, as a "plus" section and as a "minus" section of thefilter.

In the drawings:

FIGS. 1 and 1a diagrammatically illustrate the architecture of thedouble-split electrode transversal filter of the aforesaid article.

FIG. 1b illustrates the shape of a potential well formed by atransversal filter having the type of architecture shown in FIGS. 1 and1a;

FIG. 2 illustrates a first embodiment of the present invention;

FIG. 2a illustrates the shape of a potential well formed by atransversal filter having the architecture shown in FIG. 2;

FIG. 3 illustrates a second embodiment of the present invention; and

FIG. 3a illustrates a potential well of a transversal filter having thearchitecture shown in FIG. 3.

Referring to FIGS. 1 and 1a, there is shown prior art architecture for atransversal filter of the type described in the aforesaid article. Morespecifically, semiconductor surface 100 of given conductivity, such as Psilicon, includes a channel of given width defined by the edges of apair of substantially equidistant potential barriers comprised of thickoxide regions 102 and 104 which operate as channel stops. Extending thegiven width of the channel between channel stops 102 and 104 is a firstset of double-split gate electrodes 106. As clearly shown in FIG. 1a,each of double-split gate electrodes 106 includes a "plus" (+) sectionextending from channel stop region 102 to thick-oxide region 108operating as a channel stop island, a clock (CL) section extending fromchannel stop island 108 to thick oxide region 110 also operating as achannel stop island and a "minus" (-) section extending from channelstop island 110 to channel stop region 104. All the sections of all thedouble-split gate electrodes 106 shown in FIG. 1 may be comprised ofpolysilicon, with the splits themselves being formed by those portionsof channel stop islands 108 and 110 which do not underlie these sectionsof polysilicon gate electrode 106. All three of the center sections (CL)of all the first set of gate electrodes 106 have φ₁ phase clock voltagesapplied thereto. In addition, the "plus" sections of all the first setof gate electrodes 106 are applied in common through "plus" summing bus112 as an input to sensing circuit 114. All of the "minus" sections ofall of the first set of gate electrodes 106 are connected in commonthrough "minus" summing bus 116 as an input to sensing circuit 118.Differential circuit 120 produces an output from the transversal filterwhich is equal to the difference in the respective outputs from sensingcircuits 114 and 118.

Interleaved between each pair of first set double-split gate electrodes106 is a second set of non-split gate electrodes 122, shown in FIG. 1.φ₂ phase clock voltages are applied to all of second set gate electrodes122.

The arrangement shown in FIG. 1 and FIG. 1a comprises a two-phase clockvoltage CCD arrangement. Multiple-phase clock voltage CCD arrangementsthat employ more than two phases, are known in the CCD art. In suchcase, the gate electrodes are arranged in respective sets correspondingto each of the multi-phase clock voltages, with the respective membersof each set being distributed along the length of the channel ininterleaved relationship with respective members of the other sets. Inthis latter case, a plurality of non-split gate electrodes, similar togate electrode 122, of all sets but the first set would be situatedbetween each pair of first set split-gate electrodes, with a differentphase clock voltage being applied to each one of these plurality ofintervening non-split gate electrodes. For illustrative purposes, indescribing the present invention, the two-phase clock voltage CCDarrangement is assumed. However, it should be understood that theinvention is applicable to multiple phase clock voltage CCD arrangementthat employ more than two phases of clock voltage.

Referring to the embodiment of the present invention shown in FIG. 2,substrate surface 200, channel stop regions 202 and 204 and second setnon-split gate electrode 222, having φ₂ phase clock voltage appliedthereto, are respectively substantially identical in structure andfunction to substrate surface 100, gate electrode regions 102 and 104and second-set non-split gate electrode 122 of FIGS. 1 and 1a. However,the structure of each first-set split gate electrode 206, in FIG. 2, issubstantially different from the structure of each first-set split gateelectrode 106, in FIGS. 1 and 1a. First, each split gate electrode 206is a triple-split gate electrode formed of four sections, rather than adouble-split gate electrode formed of three sections. Specifically, thefour sections comprise a first CL section 208, a "plus" section, a"minus" section and a second CL section 210, extending in that order thewidth of the channel defined by the edges of channel stop region 202 andchannel stop region 204. In accordance with the principles of thepresent invention, the sum of the respective lengths of the first CLsection 208 and the "plus" section of all the first-set split gateelectrodes 206 lie on one side of the mid-line of the channel and aresubstantially equal in length to the sum of the lengths of second CLsection 210 and "minus" section of all the first-set gate electrodes206, which lie on the other side of the mid-line of the channel.Therefore, the sum of the lengths of the sections of split gateelectrode 206 lying on one side of the mid-line of the channel and thesum of the lengths of the sections of each split gate electrode 206lying on the other side of the mid-line of the channel are substantiallythe same for all first-set split gate electrodes 206 of the filter, and,hence, are independent of the particular design characteristics of thefilter. However, the relative lengths of first CL section 208 and the"plus" section and the relative lengths of the second CL section 210 andthe "minus" section do vary from one split gate electrode 206 to anotherin dependence on the particular design characteristics of the filter. Asshown in FIG. 2, φ₁ phase clock voltages are applied to all foursections of all first-set split gate electrodes 206. However, as knownin the CCD transversal filter art, the "plus" and the "minus" sectionsof a split gate electrode filter may be left floating duringcharge-transfer. In this latter case, φ₁ phase clock voltages would beapplied to only the first and second CL sections 208 and 210 of all thefirst-set split gate electrodes 206.

Another structural difference between the split gate transversal filtershown in FIG. 2 and that shown in FIGS. 1 and 1a, is the presence ineach of the three-split regions between the four sections of eachfirst-set gate electrode 206 of relatively high doping with implants ofa conductivity opposite to that of substrate surface 200. Thus,assuming, in FIG. 2, substrate surface 200 to be comprised of P silicon,each of the split regions are comprised of N⁺ implants, as indicated bythe legend "XXXX" in the drawings.

There are many benefits to be derived from employing the structure ofFIG. 2 for a split gate electrode transversal filter, rather than thatshown in FIGS. 1 and 1a. First, the respective positions of the channelstop islands 10, the polysilicon sections of first-section split gateelectrodes 106 and the metallic electrical contacts on the centrallylocated CL section of each polysilicon split gate electrode 106 varyfrom one-split gate electrode 106 to another in dependence on theparticular design characteristics of each different filter to befabricated. However, as is known in the solid-state device fabricationart, a different mask is required to define the respective positions ona chip of (1) channel stops, (2) polysilicon sections of gate electrodesand (3) metallic electrical contacts on the polysilicon sections.Therefore, the fabricated split gate electrode transversal filteremploying the structure shown in FIGS. 1 and 1a requires that three ofthe masks in the mask set be changed or modified for each and everydifferent particular design characteristics split-gate electrodetransversal filter to be fabricated.

In FIG. 2, the respective positions of all the channel stops (regions202 and 204) and the metallic electrical contacts on the polysiliconsections (including contacts 224 to first and second CL sections 208 and210) are independent of the particular design characteristics of thesplit-gate electrode transversal filter to be fabricated. Only therespective positions of the polysilicon sections of the gate electrodesand the N⁺ implants vary, in the structure shown in FIG. 2, with eachdifferent particular design characteristics of the split gate electrodetransversal filter to be fabricated. In FIG. 2, only a single mask mustbe modified, defining the positions of the sections of each one of thefirst-set polysilicon gate electrodes 206 of the filter, for eachdifferent particular design characteristics of the transversal filter tobe fabricated. The respective positions for inserting the N⁺ implants,in FIG. 2, (i.e., the position of the splits between the section of thepolysilicon gate electrode 206) are self-aligned by the earlier layingdown of the polysilicon gate electrodes in accordance with the singledifferent mask. Thus, there is a saving of two masks for each and everydifferent particular design characteristics transversal filterfabricated in accordance with the structure of FIG. 2, rather than inaccordance with the structure of FIGS. 1 and 1a.

There are additional benefits to be gained by employing the structure ofFIG. 2, rather than that of FIGS. 1 and 1a. The useful charge of a CCDsplit gate electrode transversal filter is that stored respectively inthe potential well under a "plus" section and in the potential wellunder "minus" section of a split gate electrode. In the structure shownin FIGS. 1 and 1a, both ends of both the "plus" and the "minus" sectionsof split gate electrodes 106 overlap either channel stop regions 102 or104 or channel stop islands 110. As indicated in FIG. 1b, the width of a"plus" or "minus" potential well extends from an edge of a channel stopregion 102 or 104 to an edge of a channel stop island 110. However, asalso indicated in FIG. 1b, both ends of such a potential well are notwell defined, but slope relatively gradually downward. This poordefinition is due to the presence of a channel stop at the boundaries ofthe potential well. By contrast, in the structure of FIG. 2, the ends ofa "plus" or "minus" potential well are, in effect, extremely sharp(which is desirable), as indicated in FIG. 2a. The reason for this isthat, in the structure of FIG. 2, a "plus" or "minus" potential well isbounded by the edge of an N⁺ implant, rather than by the edge of achannel stop. An N⁺ implant does not act as a channel stop, but ratherforms a PN junction with the underlying substrate that assumes apotential level substantially equal to that produced by the chargepackets partially filling the potential well underlying the split gateelectrodes adjacent the N⁺ implant. Therefore, a constant potentiallevel extends across the channel from the vicinity of channel stop 202to the vicinity of channel stop 204.

In the structural arrangements of both FIG. 2 and FIGS. 1 and 1a, it isdesirable to provide a fixed "minimum" length for the smaller of the"plus" or the "minus" sections of each individual first said split gateelectrode of any given transversal filter, and to provide a properlyweighted length for the larger of the "plus" or "minus" section of eachfirst-set split gate electrode in accordance with the particular designcharacteristics of that given filter. The reason for doing so is that itresults in the minimization of unwanted capacitance of the "plus" and"minus" sections of the split gate electrodes. In the structure shown inFIGS. 1 and 1a, only the portions of the lengths of the "plus" and"minus" sections of the split gate electrodes which lie innon-overlapping relationship with respect to the channel stop regions102 and 104 and the channel stop islands 110 contribute to the operationof the CCD transversal filter. However, the entire lengths of the "plus"and "minus" sections of the split gate electrodes contribute to theunwanted capacitance thereof. In the structure shown in FIG. 2, theentire lengths of the "plus" and "minus" sections of the split gateelectrodes contribute to the operation of the CCD transversal filter.Because no channel stops are employed in the structure of FIG. 2, the"minimum" length of the smaller of the "plus" or "minus" sections of thesplit gate electrodes can be reduced in size relative to the "minimum"length needed in the structure of FIGS. 1 and 1a (which must overlapchannel stops). This structural difference reduces unwanted capacitanceand, in addition, reduces the common mode signal at the inputs to thesense circuits.

In the structure shown in FIG. 2, the electrical contacts 226 of the"plus" and "minus" sections are positioned to overlie active potentialwell charge-transfer areas of substrate 200. This is not believed byApplicant to be in an impediment. There are those who do believe thatthe practice of placing an electrical contact directly over a potentialwell produces an undesired perturbation in the depth of the potentialwell formed under such sections. The embodiment of the present inventionshown in FIG. 3 avoids this practice.

In the structure shown in FIG. 3, a channel defined by channel stopregions 302 and 304 is divided into substantially two equal widthsub-channels by channel stop region 328 situated along the mid-line ofthe channel as shown in FIG. 3. Overlying the sub-channel betweenchannel stop 302 and 328 is "plus" section and first CL section 308 ofeach first-set split gate electrode 306, with one end of each "plus"section overlapping channel stop 302 and one end of each first CLsection 308 overlapping channel stop 328. An N⁺ implant is situatedwithin the split between a "plus" section and a first CL section 308.Overlying the sub-channel between channel stop 304 and channel stop 328is a "minus" section and a second CL section 310 have each first-setssplit-gate electrode 306, with one end of a "minus" overlapping channelstop 328 and one end of second CL section 310 overlapping channel stop304. An N⁺ implant is situated in the split between each "minus" sectionand second CL section 310. As shown in FIG. 3, electrical contacts 324of the first and second CL sections and the electrical contacts 326 ofthe "plus" and the "minus" section are situated over channel stop 302,channel stop 304 or channel stop 328, as the case may be. In all otherrespects, the structure of FIG. 3 is substantially similar to that ofFIG. 2 and that of FIGS. 1 and 1a.

The primary benefit of Applicant's invention of requiring only a singledifferent mask for each transversal filter of different particulardesign characteristics to be fabricated, is preserved by the structureof FIG. 3. However, the secondary benefits of the embodiment shown inFIG. 2 are partially lost in the embodiment of FIG. 3, as indicated byFIG. 3a. As shown in FIG. 3a, of a "plus" or a "minus" potential well ispoorly defined by a channel stop edge, as is the case in FIGS. 1 and 1a,while the other potential well end is effectively a sharply defined N⁺implant edge, as is the case in the embodiment of FIG. 2.

It is not essential that the "plus" sections overlap channel stop 302and the "minus" sections overlap channel stop 328, as specifically shownin FIG. 3. However, it is essential to the embodiment of the presentinvention shown in FIG. 3 that only one of the "plus" and the "minus"sections overlap a channel-defining channel stop, such as either channelstop 302 or channel stop 304 and the other of the "plus" and the "minus"sections overlap mid-line channel stop 328. The reasons for this isthat, in practice, it is not possible to perfectly align masks. However,it is vital that any misalignment of masks not substantially affect thevalue of the difference beween the effective lengths of the "plus" andthe "minus" sections of any split gate electrode of transversal filter.Specifically, misalignment which results in a decrease in the overlap ofa channel stop by the "plus" section of a split gate electrode must alsoresult in a substantially equal decrease in the overlap in the overlapof the "minus" section of that split gate electrode. Similarly, amisalignment which results in an increase in the overlap of the "plus"section of a split gate electrode must also result in a substantiallyequal increase in the overlap of the "minus" section of that split gateelectrode.

What is claimed is:
 1. In a CCD split electrode transversal filteroperated by multi-phase clock voltages, said filter being of a typecomprised of a semiconductor substrate surface on which a pair ofsubstantially equidistant potential barriers defines a CCD channel of agiven width, gate electrode extending said given width between saidpotential barriers, said gate electrodes being arranged in respectivesets corresponding to each of said multi-phase clock voltages with therespective members of each set being distributed along the length ofsaid channel in interleaved relationship with respective members of theother sets, certain members of a certain one of said sets each beingsplit into a plurality of separated sections arranged in serial orderacross said given width, a "plus" summing bus electrically connectingtogether first corresponding sections of the members of said certain oneof said sets, and a "minus" summing bus electrically connecting togethersecond corresponding sections of said certain members of said certainone of said sets; the improvement:wherein said certain members of saidcertain one of said sets are split into four sections having respectiveextents such that (1) the sum of the extents of the first and secondsections in serial order is substantially one-half said given width ofsaid channel and (2) the sum of the extents of the third and fourthsections in serial order is also substantially one-half said given widthof said channel, first means including contacts for electricallyconnecting each one of a given pair of non-adjacent ones of said foursections of said certain members of said certain one of said sets tosolely that one of the multi-phase clock voltages corresponding to saidcertain one of said sets, second means including contacts forelectrically connecting first corresponding ones of the remaining pairof said four sections of said certain members of said certain one ofsaid sets to said "plus" summing bus, and third means including contactsfor electrically connecting second corresponding ones of the remainingpair of said four sections of said certain members of said certain oneof said sets to said "minus" summing bus.
 2. The filter defined in claim1,wherein one end of the first section of said certain members of saidcertain one of said sets is situated contiguous to one of said pair ofpotential barriers and one end of said fourth section of said certainmembers of said certain one of said sets is situated contiguous to theother of said pair of potential barriers, wherein one end of the secondsection and one end of the third section of said certain members of saidcertain one of said sets are situated contiguous to the mid-line of saidchannel, which mid-line divides said given width in half, wherein therespective positions of the other ends of said first and second sectionsand the other ends of said third and fourth sections of each certainmember of said certain one of said sets varies from one certain memberto another in dependence on the particular design characteristics ofsaid filter, and wherein said contacts of said first and second andthird means have fixed predetermined positions on the respective foursections of each certain member of said certain one of said sets, whichfixed predetermined positions are independent of the particular designcharacteristics of said filter.
 3. The filter defined in claim 2,whereinsaid first means is electrically connected through its contacts to saidfirst and fourth sections of each certain member of said certain one ofsaid sets, said second means is electrically connected through itscontacts to one of said second and third sections of each certain memberof said certain one of said sets, and said third means selectivelyconnected through its contacts to the other of said second and thirdsections of each certain member of said certain one of said sets,wherein said semiconductor substrate, at least in the vicinity of saidsubstrate surface, is doped with carriers of a given conductivity, andwherein the respective separations between adjacent pairs of saidsections of each certain member of said certain one of said sets definerespective gaps, said gaps being implanted with carriers of aconductivity opposite to said given conductivity in sufficientconcentration to form respective PN junctions with said substrate atsaid gaps.
 4. The filter defined in claim 3,wherein said one of saidpair of potential barriers comprises a first channel-stop region at saidsubstrate surface and said other of said pair of potential barrierscomprises a second channel-stop region at said substrate surface,wherein each end portion including said one end of the first section ofsaid certain members of said certain one of said sets overlies saidfirst channel-stop region and each end portion including said one end ofthe fourth section of said certain members of said certain one of saidsets overlies said second channel-stop region, and wherein said contactsof said first means are positioned on said respective end portions ofsaid first sections and said fourth sections of said certain members ofsaid certain one of said sets.
 5. The filter defined in claim 2,whereinsaid one of said pair of potential barriers comprises a firstchannel-stop region at said substrate surface and said other of saidpair of potential barriers comprises a second channel-stop region atsaid substrate surface, wherein each end portion including said one endof the first section of said certain members of said certain one of saidsets overlies said first channel-stop region and each end portionincluding said one end of the fourth section of said certain members ofsaid certain one of said sets overlies said second channel-stop region,wherein said channel includes a third channel-stop region situatedhalf-way between said first and second channel stop regions along themid-line of said channel for dividing said channel into a firstsub-channel situated between said first and third channel-stop regionsand a second sub-channel situated between said second and thirdchannel-stop regions, wherein each end portion including said one end ofsaid second section and each end portion including said one end of saidthird section of said certain members of said certain one of said setsoverlies said third channel-stop region, and wherein said first meansincludes contacts positioned on the respective end portions of saidfirst and third sections, said second means includes contacts positionedon the end portions of the same given one of said second and fourthsections, and said third means include contacts positioned on the endportions of the other one from said given one of said second and fourthsections of said certain members of said certain one of said sets.
 6. ACCD split gate electrode transversal filter including:at least onetriple-split gate electrode comprised of four serially-orientedlongitudinal sections, wherein the sum of the lengths of the first twosections in serial order is substantially equal to the sum of thelengths of the last two sections in serial order, first means foroperating two non-adjacent ones of said four sections as solely clocksections, and second means for operating the other two of said foursections, respectively, as a "plus" section and as a "minus" section ofsaid filter.
 7. The filter defined in claim 6,wherein said first meansoperates the first and the fourth ones of said four sections in serialorder as solely said clock sections.
 8. The filter defined in claim6,wherein said first means operates the first and third ones of saidfour sections in serial order as solely said clock sections.